or SOUTHERN INDIA. 229 



few have been made known since. I am acquainted with at least two new species, 

 one from the Andamans and the other from Singapore, but very likely several 

 others will be found in our seas. 



The genera arrange themselves according to their geological age as follows : — 



1. DlcerocarcUum, Stopp., 1865, (Pal. Lomb., 3™ ser., Appendix p. 248). 

 Shell, as far as known, equivalve, with strongly prominent and spirally twisted 

 and carinated beaks without any perceptible j)lace of attachment, solid and usually 

 of large size ; a free projecting lamina extends from the beaks to the hinge, which 

 is provided in the left valve with a strong cardinal tooth corresponding to a 

 pit in the right valve, and besides this there are two elongated laminar teeth in 

 each valve adjoining or superseding the former ; ligament external, situated in 

 a groove along the upper and part of the posterior edge ; the shell consists of an 

 outer lamellated and inner fibrous layer ; type, D. Jani, Stopp. 



This genus is as yet only known from a few species represented in the so- 

 called Infra-lias or upper triassic beds of the southern declivities of the Alps. 

 I have described from the same geological horizon in the Himalayas a new species, 

 D. Simalmjense, (Mem. Geol. Survey of India, vol. v, p. 63). 



2. Dlceras, Lamck., 1801. Shell sub-equivalve, solid, with strong prominent, 

 spirally incuiwed umbones, one of which is sometimes attached ; a more or less 

 distinct internal ridge rims internally from the umbones near the i:)osterior margin 

 of the shell and bounds the posterior muscular impression in front; left valve 

 with a strong sub-anterior blunt cardinal tooth fitting into a fosset of the right 

 valve which has posteriorly a very strong broad cardinal tooth ; besides that there 

 is in each valve a small posterior lateral tooth adjoining the muscular impression ; 

 this last is elongated, the anterior one more transversally (or longitudinally) oval ; 

 pallial line entire ; ligament external, situated in a slight groove beginning at the 

 umbones and extending along the posterior margin as far as the lateral tooth ; it 

 is supported by strongly thickened fulcra. Type, Dlceras arietiiium, Lam. 



The species are not numerous in the upper Jurassic and become rather 

 rare already in the cretaceous deposits, but there appears to be a considerable difii- 

 cvilty in distingviishing from them some of the species belonging to the next allied 

 genera, particularly when the fossil forms are not well preserved. 



3. Mequienia, Matheron, 1842, (Cat. Meth. et Descript. des corps org. foss. du 

 Dept. des B. du Rhone, &c., p. 102). Shell inequivalve, lower valve large, spiral, 

 contorted and adherent, upper valve smaller, more or less flattened, but always with 

 a spiral beak ; an internal posterior rib is usually seen in the lower valve from the 

 umbo extending to the postero-inferior margin and bounding near it the posterior 

 muscular impression ; the hinge consists in the lower valve of a more or less pro- 

 minent marginal, lamelliform tooth, fitting into a cavity of the smaller valve which 

 has one tooth on either side (above and below) of it ; there is besides a posterior small 

 lateral tooth in each valve present ; paUial line entire ; ligament external, situated in 

 an umboual groove of the lower valve and extending to the hinge, and then along its 

 postero-superior margin. Type, B. ammonia, Goldf. The hinge of this genus most 



